With the development of display technology, liquid crystal display devices are more widely used. Generally, a liquid crystal display device employs an edge type backlight device which typically comprises a light guide plate and a light source at a side of the light guide plate. The material of the light guide plate generally absorbs selectively the blue light in the white light, so that more blue light remaining at the light incoming side of the light guide plate than that at the side far from the light source. Thus, the light incoming side of the light guide plate is blueish while the side far from the light source is yellownish, resulting in a color shift phenomenon, and thereby degrading the image quality of the display device.